Rice—A Step Toward Use of Allelopathy

house and field experiments. Research to accomplish the above goals cannot be carried out in one laboratory, Rice (Oryza sativa L.) allelopathy has been on the research agenda and needs active collaboration among a wide range of for a decade. Now it is important to step back and look at its progress to enable priority setting for future research. This paper aims to do scientists, including biologists, ecologists, agronomists, so primarily using the following five-step protocol for allelopathy natural product chemists, plant physiologists, and genetresearch: (i) carrying out laboratory, greenhouse, and field studies to icists. To a large extent, research on allelopathy in rice illustrate the effect of released allelochemicals; (ii) isolating, identihas endeavored to achieve these goals. Rice has thus fying, and characterizing allelochemicals; (iii) establishing a correlabecome a model plant and an example of successful tion between growth inhibition and allelochemicals; (iv) performing progress toward using allelopathic rice cultivars for genetic mapping of quantitative trait loci (QTLs) correlated with weed management. This paper briefly summarizes reallelopathy; and (v) breeding for allelopathic cultivars tested for comcent progress in rice allelopathy, and identifies research petitive ability in greenhouse and field experiments. Recent research components that still need to be studied to provide on rice allelopathy has resulted in the following research milestones: guidance for future research. • There is large variation in allelopathy among rice cultivars. • Allelopathy plays a role under field conditions. ABOUT RICE ALLELOPATHY • Allelopathic rice can suppress both monoand dicot weed species. Rice allelopathy has been a subject of continued re• Progress has been made in identifying rice allelochemicals. search for a decade and progress has been made in a • Quantitative trait loci correlated with allelopathy have been idenrange of fields, thus adding to the understanding of it tified. (for a review of rice allelopathy, see Olofsdotter et al., This paper discusses the progress made in recent years and suggests 1995 and Olofsdotter, 1998). All biological research is some direction for future research. a succession of processes of success and failure to test hypotheses that can increase our understanding of biological processes and thereby advance research. These I the competitive ability of crops might restepwise processes can be evaluated by knowledge mileduce dependency on herbicides. Attempts to instones, which are discussed for rice allelopathy. crease competitive ability, however, have had limited success and no crop cultivar has been released with There is Large Variation in Allelopathy superior competing ability as a marketing argument. among Rice Cultivars The lack of progress in creating competitive cultivars To quantify allelopathy in rice, several laboratory and might be due to the complexity and lack of understandfield screening procedures have been developed and ing of the components and mechanisms of competition. tested (Fujii, 1992; Dilday et al., 1998; Navarez and OlofsOnly when we have understood and are able to optimize dotter, 1996; Olofsdotter and Navarez, 1996). The conseveral competition components will a truly good comclusions from all these studies were that there are differpetitor be a reality. The importance of chemical interferences in allelopathic potential among rice cultivars, ence, including allelopathy, in crop competition has ofexpressed as a reduction in root growth of test plants ten been discussed (Rice, 1995). More recently, Wu grown in association with rice in the laboratory or as a et al. (1999) reviewed the possibilities of genetically weed reduction (dry matter and density) in the field. improving crops with allelopathic potential and stated For the field screening, however, it is important to note that allelopathy can play an important role in future that allelopathy is difficult to distinguish from competiweed management. To convincingly demonstrate the tion and therefore such screening must be compared possibilities of using allelopathy, focused research is with data where resource competition can be eliminated needed. For this, the following are needed: (i) laboraas a factor in the experiment. Olofsdotter et al. (1999) tory, greenhouse, and field studies carried out to illuscorrelated screening results from the laboratory with a trate the effect of released allelochemicals; (ii) allelorange of competition components, measured in the field, chemical isolation, identification, and characterization; and claimed that allelopathy can explain 34% of the (iii) a correlation established between growth inhibition reduction in total weed dry weight 8 wk after seeding. and allelochemicals; (iv) genetic mapping of QTLs corTable 1 presents selected data from this study including related with allelopathy; and (v) breeding for allelobarnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] pathic cultivars tested for competitive ability in greenplant dry weight 8 wk after seeding, tillering, rice and weed height, the barnyardgrass root length in laboratory International Rice Research Institute, APPA Division, P.O. Box 3127, screening, and a cultivar ranking. Plant height is often MCPO, 1271 Makati City, Philippines, and Weed Science, Royal Veterinary and Agric. Univ., Agrovej 10/2630 Toastrup, Denmark. Redescribed as one of the most important factors for total ceived 30 Nov. 1999. *Corresponding author ([email protected]). Abbreviations: QTLs, quantitative trait loci. Published in Agron. J. 93:3–8 (2001).